The present invention relates to open celled foams and, in particular, to open celled polyimide-polyamide foams wherein the ratio of imide groups to amide groups is greater than 1 to 1 (hereinafter "polyimide-polyamide foams") and a process for forming such foams which utilizes unique nonionic, organic surfactants.
An open celled polyimide-polyamide foam formulation comprises reactants or monomers, a catalyst, surfactant, and a cell opener. The surfactant is primarily responsible for producing a foam bun having a good height; small, uniform cells; and a low thermal conductivity. Historically, polyimide-polyamide foams, having a ratio of imide groups to amide groups that is greater than 1 to 1 and no greater than 19 to 1 have been made with silicone surfactants.
While these silicone surfactants have functioned satisfactorily when making open celled polyimide-polyamide foams having densities of 0.6 pounds per cubic foot (0.6 pcf) or greater, there has been a need to have better performing surfactants for producing open celled polyimide-polyamide foams having densities of less than 0.6 pcf, such as open celled polyimide-polyamide foams having densities of 0.3 pcf. The task of forming open celled polyimide-polyamide foams having densities of about 0.3 pcf with good heights, small uniform cells, and good thermal conductivities, is much more difficult than forming open celled polyimide-polyamide foams having densities of 0.6 pcf or greater. To decrease the density of the polyimide-polyamide foams from 0.6 pcf to 0.3 pcf, twice the amount of gas must be released from the reactants which must contain all of the gas to be released. If any of the gas (carbon dioxide) is released prematurely, the density of the polyimide-polyamide foam formed will be greater than desired. In addition, due to the amount of gas released in making such a low density polyimide-polyamide foam, it is much more difficult to control the size and uniformity of the cell structure so that the polyimide-polyamide foam formed has small, uniform cells.
It was discovered that the quality of low density open celled polyimide-polyamide foams correlates with the type of surfactant used in the reactant mixture. The reactant mixtures used to form the open celled polyimide-polyamide foams of the present invention utilize nonionic organic surfactants which outperform other surfactants, such as the silicone surfactants previously used in the production of such open celled polyimide-polyamide foams. It was further discovered that nonionic organic surfactants, having hydrophilic/lipophilic balances of at least 12 and preferably 15 or greater (HLB values of at least 12 and preferably 15 or greater), formed the best quality low density, open celled polyimide-polyamide foam buns, e.g. open celled polyimide-polyamide foam buns having densities of about 0.3 pcf, with good heights, small uniform cells, and low thermal conductivities. In other words, the cell structure and the height of the foam buns produced correlated with the HLB value of the nonionic, organic surfactants. The hydrophilic portions of these surfactants are made up of ethylene oxide and the lipophilic portions of these surfactants are made up of alkyl phenols, fatty acids, propoxylated adducts, etc. Tests showed that nonionic organic surfactants with lower HLB values, i.e. with lower percentages of ethylene oxide, produced the worst looking foam buns and that the higher the HLB value of the surfactant used in the reactants, i.e. the greater the percentage of ethylene oxide in the surfactant, the better the foam buns looked.
The reactants, with the nonionic organic surfactants, used to form the low density polyimide-polyamide foams of the present invention, also perform well in forming higher density, open celled polyimide-polyamide foams having densities ranging up to 6.0 pcf. In fact, tests have shown that 0.6 pcf open celled polyimide-polyamide foams made with reactants using silicone surfactants exhibit a thermal conductivity of about 0.31 BTU-inch per ft.sup.2 -hour-.sup.0 F while 0.6 pcf open celled polyimide-polyamide foams made with reactants using the nonionic, organic surfactants of the present invention had thermal conductivities of 0.29 BTU-inch per ft.sup.2 -hour-.sup.0 F or less.